1. Field of the Invention
This invention relates to pressure-sensitive copying systems, e.g., the kind in which a substantially colorless color former (dye) held within microcapsules is reacted, upon rupturing of the microcapsules by an applied pressure, with a co-reactant material to form distinctive colored marks. More particularly, the present invention relates to improved dye solvents useful in pressure-sensitive copying systems.
2. Description of the Prior Art
In one conventional pressure-sensitive copying system, the microcapsules are carried on one surface of a transfer sheet, referred to as a CB (coated back) sheet and the co-reactant material is carried on one surface of a record sheet, referred to as a CF (coated front) sheet. In another embodiment, the microcapsules and the co-reactant material are carried on the same surface of a single sheet. In systems for making a plurality of copies, intermediate CFB (coated front and back) sheets are provided. The sheets are usually made of paper.
Most known CB sheets carry a coating of microcapsules, which may be separate or in capsular units, i.e., clusters of capsules. Each microcapsule comprises a wall of hydrophilic colloid material such as gelatin, containing a substantially colorless chromogenic material (color former) of basic reactant chemical properties which, in use, contacts and is colored by a co-reactant material.
The co-reactant material is typically a finely divided acidic compound which is also substantially colorless in its natural form. Commonly used co-reactant materials include organic polymers and inorganic clays which are applied to the CF sheet in a suitable paper coating binder material such as starch, casein, polymer or latex.
Distinctive colored marks occur on the CF sheet following rupture of the microcapsules through localized pressure from writing, typing or printing on the noncoated front surface of a CB sheet which is positioned with its coated back surface in contact with the coated front surface of a CF sheet.
The substantially colorless color former produces color only under acidic conditions, that is, upon contact with the acidic co-reactant of the CF sheet. The color former is always dissolved in a solvent and, in many cases, is diluted with kerosene or the like. It is therefore important that the color former solution possess the required physical and chemical properties.
Generally desirable properties of the color former solutions are that it be easily encapsulated by conventional techniques; that it have good shelf life in the encapsulated form; and that it be stable at moderately elevated temperatures. It is also important that the mark produced as a result of the reaction between the color former and the co-reactant develop rapidly, be fade resistant and be resistant to bleeding or feathering as a result of capillary action or other surface phenomena.
The dye solvent (color former solvent) functions to provide a carrier for the color former and a medium for the reaction between the color former and the acidic co-reactant material. The solvent must be capable of holding the color former in solution within the microcapsule, of carrying the color former to the sensitized surface of the CF sheet when the microcapsule is ruptured, and of promoting or at least not inhibiting color development with the co-reactant. In addition, since inadvertent rupture of the microcapsule is possible by careless handling, the solvent must be noninjurious to skin, clothing or environment.
The solvent is an important factor in determining the performance of the pressure-sensitive copying system in terms of stability of the sheets to heat and storage time, rate of color development, extent of color development, and durability of image. Certain prior art dye solvents have exhibited adequate print speed and color intensity on the widely used phenolic resin-coated CF sheets. In some cases, however, objectionable odors in the copying system have been ascribed to the dye solvent itself. Such odors obviously detract from commercial acceptance of such copying systems even though the dye solvent performance is otherwise superior.
While considerable care is naturally given to avoidance of dye solvents having marginal or detrimental odor properties, there are several reasons why the selection process is neither orderly, predictable or scientific. For example, the inherent odor characteristics of a given aromatic hydrocarbon designated as a primary dye solvent may either be improved or worsened depending upon the type and quantity of diluent employed therewith.
A given aromatic hydrocarbon may have an odor deemed acceptable by average sensory standards yet may cause discomfort in a poorly-ventilated room containing massive quantitities of pressure-sensitive paper using that same hydrocarbon as the dye solvent. Thus, the odor effects become cumulative especially in areas where these paper systems are stored in permanent files. Even the use of odor maskants has, in some cases, been found to be an ineffective corrective measure.
Many nonhalogenated aromatic hydrocarbons are known to the art as dye solvents for pressure-sensitive copying systems. Among these are diaryl alkanes, triaryl dialkanes, alkylated biphenyls, alkylated terphenyls, partially hydrogenated terphenyls, alkylnaphthalenes, benzylnaphthalenes and benzyl aryl ethers. It is apparent from the prior art, however, that the guidelines for odor classification of the aforementioned aromatic hydrocarbons are neither well established nor widely applicable.
U.S. Pat. No. 4,003,589, which issued Jan. 18, 1977, discloses certain alkylnaphthalenes said to be useful as dye solvents. In defining the alkylation levels for obtaining optimum performance, the patentees state that the odor will be undesirable if the total number of carbon atoms in the substituted alkyl groups is smaller than 4.
In U.S. Pat. No. 3,836,383, which issued Sept. 17, 1974, there are disclosed certain diphenylalkanes useful as dye solvents. The patentees state that the usual aromatic hydrocarbons do not satisfy the odor requirements established for a suitable dye solvent. Each of the diphenylalkane compounds exemplified in U.S. Pat. No. 3,836,383, regardless of type or location of alkyl substitution in the rings, was said to not have the unpleasant smell associated with polychlorinated diphenyls of the prior art.
U.S. Pat. No. 3,996,405, which issued Dec. 7, 1976, discloses certain ethyldiphenylmethanes useful as dye solvents. The ethyl group in the one benzene ring is said by the patentee to be attachable at the ortho, meta or para positions with equal performance. Thus, no recognizable advantage was seen through selective isomer positioning.
U.S. Pat. No. 3,627,581 which issued Dec. 14, 1971, discloses isopropylbiphenyl as a dye solvent. The isopropyl group, according to the patentee, may be attached to the benzene ring at the ortho, meta or para positions. Some performance preference was seen, however, for the meta and para isomers versus the ortho isomer of isopropylbiphenyl. No distinction was stated, nor did the patentee give attention to, the odor characteristics of isomer variations of isopropylbiphenyl.
Thus, while certain classes of aromatic hydrocarbons have recently been identified as outstanding performers as dye solvents in pressure-sensitive copying systems, there still remains a lack of understanding of routes to odor improvement. Upgrading of print intensity and fade resistance is exemplified in aforementioned U.S. Pat. No. 3,996,405 wherein ethyldiphenylmethane is said to be superior to isopropylbiphenyl, the latter described in U.S. Pat. No. 3,627, 581. Odor improvements in alkylated diphenylmethanes such as ethyldiphenylmethane would constitute a step forward in the dye solvent art.
It is an object of the present invention, therefore, to provide certain alkylated diphenylmethanes which have been found to exhibit surprisingly superior odor characteristics together with print performance essentially equivalent to ethyldiphenylmethane. Further objects of this invention will become apparent from the following description and examples.